The present invention relates to a polarization diversity optical receiving method, its apparatus, an optical transmission apparatus, and an intermediate-frequency stabilization method which are simpler in configuration as compared with the prior art and which are suitable for cost reduction.
From the view-point of combining method of two branches outputs, polarization diversity optical receiving methods and their apparatuses used for coherent optical transmission are broadly classified into intermediate-frequency combining type and baseband combining type. Conventional polarization diversity optical receiving methods and their apparatuses of intermediate-frequency combining type are discussed in IOOC '83, 30C3-2, pp. 386 to 387, 1983, for example. Further, conventional polarization diversity optical receiving methods and apparatuses of baseband combining type are discussed in Meeting of Technical Group on Optical and Quantum Electronics, The Institute of Electronics, Information and Communication Engineers, OQE 88-70, 1988, for example.
Conventional intermediate-frequency stabilization methods are discussed in Meeting of Technical Group on Optical and Quantum Electronics, The Institute of Electronics, Information and Communication Engineers, OQE 88-85, 1988, for example.
In the above described conventional techniques of baseband combining type, two demodulators with the same characteristics are indispensable in order to convert two intermediate-frequency (hereafter referred to as IF) signals to baseband signals, respectively. An infinite quantity (dB) of dynamic range is required for each demodulator. ("Dynamic range" refers to a range of input signal power over which demodulators normally operate. The dynamic range is usually represented by the ratio of the maximum value of power to its minimum value.) This is because the proportion of the power of the optical signal inputted to one branch varies from 0% to 100% because of polarization fluctuations of the optical signal. On the other hand, in case of IF combining type, one modulator is needed, and the power variation of the inputted signal is as small as 3 dB. The IF combining type is thus free from problems a companed by the baseband type. However, an automatic phase controller needed to add two IF signals under the state that phases nearly agree with each other becomes very complicated in configuration because the phases of semiconductor lasers used as an optical transmittor and an optical local oscillator are very unstable. (In typical semiconductor laser, phase noises referred to as spectrum line width range from several MHz to several tens MHz.) This results in problems that the polarization diversity optical receiving apparatus becomes large in size, complicated in configuration and high in cost.
In the above described prior art of the IF stabilization method, an oscillator and a modulator are needed, and an apparatus for implementing stabilization thus becomes high in cost, resulting in a problem.